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Female chimps are more promiscuous than their human counterparts

Violent male chimps have provided new insights into the cause of wife beating, holding up a kind of mirror to help scientists understand the roots of domestic violence.

Male chimpanzees can be highly aggressive toward female group members, even using branches as clubs to beat them. Research carried out over many years in the Kibale National Park in Uganda now links this to female promiscuity and suggests that there would be more attacks on women if human society was as promiscuous as ape society.

However, another conclusion of today’s study is that because men play a role in bringing up children, unlike male apes, they are aggressive towards women who they suspect of cheating on them, since they may end up having to raise another man’s child.

Dr Martin Muller of Boston University and his colleagues at Harvard report today in the Proceedings of the Royal Society, Biological Sciences how they used data from Kibale to test the idea that male aggression is triggered by sexual coercion - ensuring that females only mate with them and pass on their genes.

“Our results supported this idea, as males were most aggressive toward the most fecund females, and mated more with the females they were most aggressive towards,” said Dr Muller. “We conclude that male-female aggression represents an evolved strategy to constrain female sexuality, which in chimpanzees has evolved to favour promiscuous mating.”

Human and ape societies differ greatly in this respect.

“Chimpanzee females are highly promiscuous, and they seem to want to mate with all the males in their communities. Most of the aggression that we observed in our study seemed to be directed toward females to prevent them from mating with other males.

“Thus, sexual coercion in chimpanzees more commonly involves males using aggression to constrain female sexuality, rather than to overcome female reluctance. Males are basically trying to force females into exclusive mating relationships. This is thus much more similar to wife-beating in humans than it is to rape, for example.”

“It’s a sign of our relative monogamy that there is relatively little wife beating (compared to chimpanzees anyway),” he added. “But there is a similarity, in that when men do beat their wives, it tends to be because they are trying to control them (that is, violence against partners is commonly due to sexual jealousy or the perception of infidelity).”

As the January sales draw to a close, scientists have revealed that humans are not alone in their ability to hunt for bargains.

Researchers at the University of St Andrews have discovered that baboons are also skilled shoppers with the ability to plan ahead and search for a good deal.

Dr Rahel Noser and Prof Richard Byrne of the University’s School of Psychology witnessed baboons in South Africa apparently planning ‘shopping trips’ (in the baboons’ case, hunting for food) by comparing the best available goods on offer before selecting the best route to take to get to them.

By tracking the animals with GPS, they found that the primates used their imagination to consider what else might be available in other areas of the woodland before making their choice.

Dr Noser said, “Baboons are evidently able to compare different possible places in their imagination, which means that they can plan ahead and choose a route that is efficient in getting to their next place to visit.”

Professor Byrne said, “A smart shopper plans their route in advance, deciding which of several shops selling similar goods to visit, and working out the most efficient pathway among the various chosen shops to save time and effort. We may think little of this everyday skill, but it is widely believed that non-human animals cannot imagine what is ‘out of sight’ or ‘in the future’.

“Many researchers have suggested that this kind of skill is beyond the mental capacity of non-human animals, and that their day-to-day foraging is driven only by environmental features they detect directly. For example, they might head for the smell of a tree full of ripe fruit, or the sight of tall trees that indicate water.

“However we found that these African baboons were not only led by external stimuli but were also able to imagine several out-of-sight places, compare what is available at each of them, and choose the route that minimises effort. Some primates, at least, seem to be good shoppers.”

Prof Byrne concluded: “Baboons do not seem to have anything much like a real ‘map in the head’, rather their knowledge is an interconnected network of well-known paths. They apparently see the African bush as if it were rich tangle of familiar streets.”

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Librarian's Comment: I served as the primary domestic violence prosecutor in Jackson County, Oregon for a little over a year during the early nineties. Jackson County had a population of around 140,000 people, and virtually every one of them has white skin. In the nineties, unemployment was very high, and domestic violence was booming. Week after week, the Medford Police Department and Jackson County Sheriff's Office arrested dozens of men for beating their wives and kids, and dumped the arrest reports in my inbox.

Did I learn anything about why guys were beating their wives, girlfriends, and kids? Well, I read at least 400 domestic violence police reports, and charged at least three-hundred and fifty defendants during that thirteen-month period, but I can't recall once, during that whole time, reading a police report that said the cause of the violence was female promiscuity, or even the male partner's "perception" of it.

Rather, the impression I got was that hundreds or thousands of men in the county were nursing rage born of economic and personal humiliation suffered at the hands of bullies at school and in the workplace, and that when they got home, they'd try to unwind with booze, but the rage would spill over, and they'd strike out at their loved ones. These cases were unbelievably sad, just awful domestic tragedies, complete with women pleading for mercy for the fellows who'd used their bodies for boxing practice, and guys who still tried to sell me some justification for their awful conduct.

My word to the offenders was simple -- I too had been raised in a society where domestic violence was considered a private matter, but those days were over, just like the days of legal racial discrimination were also over. There were now consequences to pay, and it was my job to impose them. They could shape up, learn anger management, or just take a walk instead of taking a swing, or I'd be sure to make arrangements for them to enjoy the publicly-provided accommodations in the jail.

This actually had a good effect on more than one offender -- at least I never had to file a probation violation on them. I don't think any deep pathology was involved in those cases. Men simply needed to hear a clear message from the law -- hit somebody, go to jail, no excuse just because they're your lover or child. Legal proscription and swift punishment deter much human misconduct, at least in the wilds of Southern Oregon.

I'm not a zoologist, so I can't say much about chimpanzee domestic violence. I only know the human variety, and I'd definitely say that 90% of the time, "promiscuity" and "sexual jealousy" have nothing to do with it. Rather, human domestic violence arises because society not only tolerates violent male inclinations, it promotes them through game and video violence. Thus, society empowers men to strike their mates and children, sometimes unto death. Domestic violence laws are a feeble defense against society's de facto embrace of male violence, that shows no sign of slackening.

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An aggressive male chimpanzee looks on from behind a bush. A 2014 study has found that male chimpanzees that are more aggressive to females sire more offspring, suggesting that the trait may have an evolutionary basis. Credit: Ian C. Gilby

Male chimpanzees that wage a campaign of sustained aggression against females sire more offspring than their less violent counterparts, new research finds.

The results suggest that such nasty behavior from males evolved because it gave the meanest males a reproductive advantage, said study co-author Ian Gilby, a primatologist at Arizona State University in Phoenix.

This chimpanzee behavior could also provide some insight into the roots of sexual aggression in men.

"It is possible that in our early ancestors there may have been an adaptive value to male aggression against females," Gilby said.

Sex, chimpanzee style

The battle of the sexes is supercharged in the chimpanzee world. Males charge at females, rip out their hair and kick, slap or beat them. Males often kill the babies of rivals to increase the availability of females to mate again.

"Male chimpanzees can be really horrible to females," Gilby told Live Science.

To deal with this behavior, female chimpanzees play a delicate balancing act. They mate with almost all the males in a troop to create uncertainty as to who's the father of the offspring. At the same time, females want to mate with the highest-quality males when they are at their most fertile, upping the chances of producing fit offspring, Gilby said.

Sustained aggression

From an evolutionary perspective, coercive sex in the animal kingdom may be advantageous because it allows otherwise undesirable males some chance of passing on their genes. But sexual aggression in male chimpanzees isn't directly parallel to rape, because it typically takes place at times distant from copulation. Female chimps also mate with multiple males anyway, Gilby said.

To understand the roots of this behavior, Gilby and his colleagues recorded instances of male-on-female violence in a troop of chimpanzees living in Gombe Stream National Park in Tanzania. The researchers studied violence that occurred both when the females were sexually receptive, or swollen, and when they were not. The team then compared that information with paternity tests on all the offspring born since 1995.

Chimps have a strict male dominance hierarchy, and more-dominant males generally engage in a greater amount of gendered aggression. But even when taking this into account, the team found that aggression increased a male's chances of siring offspring — regardless of whether the chimp was more or less dominant.

The sustained intimidation in which chimps engage, which has some parallels to human behaviors such as stalking or domestic violence, is a form of mate guarding. The behavior may make female chimps less likely to sneak off with a partner of her choosing during her most fertile times, Gilby said.

Human behaviors?

Though the findings are in chimpanzees, they lend credence to the notion that male sexual aggression in humans may have some genetic or evolutionary basis, Gilby said.

On the other hand, drawing parallels can be perilous. Humans diverged from chimpanzees at least 7 million years ago, and the human mating system looks very different from chimps' violent, multi-male, multi-female system. Humans form pair bonds and have varied and complex mating strategies and behaviors. And most men aren't brutes to their partners.

"We definitely don't mean to say this excuses or fully explains violence in this way in humans," Gilby said.

The study's findings may provide fodder for a long-standing debate in evolutionary biology about whether rape and sexual aggression are evolutionarily advantageous in humans, said William McKibbin, an evolutionary psychologist at the University of Michigan–Flint, who was not involved in the study. No studies in humans have ever shown that rape increases reproductive success, he added.

But even if such behaviors have evolutionary roots, "this is not to say that sexual coercion or rape is natural and therefore good, or that because it's in our genes, there's nothing we can do about it. Far from it," McKibbin told Live Science. "If we can understand the biological underpinnings of abhorrent behaviors like this, we will be better able as a society to decrease the frequency of these horrible behaviors."

The findings were published today (Nov. 13) in the journal Current Biology.